Wire electric-discharge machining method and device

ABSTRACT

A wire electrical discharge machining apparatus for machining a workpiece ( 2 ) by supplying discharge energy to a gap between a wire electrode ( 1   a ) and the workpiece ( 2 ) by a working-electric-power source ( 16 ) having a dc power supply ( 17 ) and switches ( 19   a   , 19   b   , 19   c , and  19   d ) for switching an output voltage of the dc power supply ( 17 ), and by relatively moving the wire electrode ( 1   a ) and the workpiece ( 2 ) by a positioning device that includes: a controller ( 20 ) which, during machining in a gas, controls the turning on and off of the switches ( 19   a   , 19   b   , 19   c , and  19   d ) so that the voltage is set to reverse polarity whereby the polarity of the wire electrode ( 1   a ) is made positive and the polarity of the workpiece ( 2 ) is made negative.

TECHNICAL FIELD

The present invention relates to improvements of a method of and anapparatus for wire electrical discharge machining for machining aworkpiece by generating electric discharge across a gap between a wireelectrode and the workpiece.

BACKGROUND ART

Electric discharge machining has established a solid position as amachining technology for such as dies and molds, and has beenextensively used in the field of die and mold machining in theautomobile industry, the household electrical appliance industry, thesemiconductor industry, and the like.

FIG. 5 is an explanatory diagram of the mechanism of electricaldischarge machining. In the drawing, reference numeral 1 denotes anelectrode; 2, a workpiece; 3, an arc column; 4, a working liquid; and 5,machining debris produced in electrical discharge machining. Removalmachining based on the electric discharge in the workpiece 2 progresseswhile repeating the cycle of the following steps (a) to (e)(corresponding to steps (a) to (e) in FIG. 5): Namely, these steps are(a) the formation of the arc column 3 due to the generation ofelectrical discharge, (b) the local fusion and the vaporization of theworking liquid 4 due to the thermal energy of electric discharge, (c)the generation of an explosive force of vaporization of the workingliquid 4, (d) the scattering of fused portions (machining debris 5), and(e) cooling, solidification, and recovery of insulation in the gapbetween the electrode 1 and the workpiece 2 due to the working liquid.

This invention concerns wire electrical discharge machining which isused in boring, cutting, and the like in electrical discharge machiningwhich effects removal machining of a workpiece by heating and fusing theworkpiece by pulse-like discharges. In particular, there has been agrowing demand for higher precision in the wire electrical dischargemachining, and high machining accuracy on the order of 1 to 2 μm orthereabouts has come to be required in the machining of high-precisiondies and molds used in the semiconductor industry and the like.

FIG. 6 is an explanatory diagram illustrating an example of themachining process of wire electrical discharge machining. In thedrawing, reference numeral 1 a denotes a wire electrode; 2, theworkpiece; 4 a, water, i.e., a working liquid; and 6, an initial hole.The part (a) of FIG. 6 shows the state of a first cut which is roughmachining, the part (b) of FIG. 6 shows the state of a second cut whichis semi-finish machining after rough machining, and the part (c) of FIG.6 shows the state of a third cut which is final finish machining.

The example of the machining of the first cut in the part (a) of FIG. 6shows machining in which the wire electrode 1 a is passed through theinitial hole 6, and the workpiece 2 is bored. In the case of such afirst cut, since the surface roughness and accuracy of the machinedsurfaces of the workpiece are finished in subsequent machining, verystrict surface roughness and accuracy are not required so much, and itis important to increase the machining speed, in particular, so as toimprove productivity. In wire electrical discharge machining, in orderto increase the machining speed, the water 4 a is powerfully sprayed tothe gap between the wire electrode 1 a and the workpiece 2 so as toefficiently discharge the machining debris from the gap. In addition, inorder to eliminate the unevenness of the application of the water 4 a tothe gap and prevent the disconnection of the wire electrode 1 a, amethod is adopted in which the water 4 a stored in an unillustratedworking tank and the workpiece 2 is immersed in it.

In the above-described conventional wire electrical discharge machining,the machining after the first cut (the part (a) in FIG. 6), such as thesecond cut (the part (b) in FIG. 6) and the third cut (the part (c) inFIG. 6), is also effected in the water 4 a, i.e., the working fluid.

FIG. 7 shows examples of the voltage and the current waveform in the gapbetween the wire electrode 1 a and the workpiece 2. In the drawing, Vdenotes a gap voltage; I, a gap current; and t, time. The state at atiming T1 in FIG. 7 is a state in which the voltage is applied acrossthe gap. When a voltage is applied across the gap, a force acts in whichthe positive polarity and the negative polarity are attracted towardeach other, so that the wire electrode 1 a having small rigidity ispulled toward the workpiece 2 side by this electrostatic force. Thiscauses the vibration of the wire electrode 1 a, so that there has been aproblem in that high-accuracy machining is made difficult due to suchvibration.

In addition, the state at a timing T2 in FIG. 7 is a state in which theexplosive force of vaporization of the working liquid has been generateddue to the discharge energy (e.g., the part (c) in FIG. 5), wherein alarge force acts on the wire electrode 1 a in a direction opposite tothat of the workpiece 2 due to the explosive force of vaporization ofthe working liquid, so that vibrations occur. There has been a problemin that irregularities occur in the shape of the workpiece 2 due to suchvibrations, which leads to the deterioration of the accuracy.

In the semiconductor industry and the like, which are the fields ofapplication of wire electrical discharge machining, in the machining ofsuch as a die for IC leadframes, applications are increasing in whichextremely high accuracy and very smooth surface roughness are requiredfor workpieces whose form accuracy is 1 μm and whose surface roughnessis 1 μm Rmax or less. In such uses, in particular, the above-describedproblem ascribable to the vibration and the like of the wire electrodehas been noticeable.

As a measure for overcoming such problems of wire electrical dischargemachining in such a working liquid, a technique concerning aerial wireelectrical discharge machining has been disclosed in which wireelectrical discharge machining is performed in the atmosphere without aworking liquid interposed in the gap between the wire electrode and theworkpiece (Adachi, Tokyo University of Agriculture and Technology, etal.: “Attaining High Precision in Second Cuts by Aerial EDM,” Die & MoldTechnology, Vol. 14, No. 7, 1999, p. 154, The Nikkan Kogyo Shimbun,Ltd.). However, although it is disclosed that accuracy in thestraightness of cut surfaces of workpieces is improved by wireelectrical discharge machining in the atmosphere, no disclosure is givenof a specific configuration and the like concerning a measure for copingwith applications in which higher accuracy is required and applicationsin which high quality of the workpiece surface is required.

DISCLOSURE OF THE INVENTION

This invention has been devised to overcome the above-describedproblems, and its object is to provide a method of and an apparatus forwire electrical discharge machining which are suitable for high-accuracymachining and high-quality machining.

The method of wire electrical discharge machining in accordance withthis invention is a method of wire electrical discharge machining formachining a workpiece by generating electric discharge in a gap betweena wire electrode and the workpiece, wherein machining is effected bysupplying to the gap a voltage of reverse polarity in which the polarityof the wire electrode is made positive and the polarity of the workpieceis made negative, in the atmosphere or while supplying a pressure gas tothe gap.

In addition, the method of wire electrical discharge machining inaccordance with this invention is a method of wire electrical dischargemachining for machining a workpiece by generating electric discharge ina gap between a wire electrode and the workpiece, comprising: a firststep of effecting rough machining in a working fluid; and a second stepof effecting finish machining in the atmosphere or while supplying apressure gas to the gap, wherein, in the second step, machining iseffected by supplying to the gap a voltage of reverse polarity in whichthe polarity of the wire electrode is made positive and the polarity ofthe workpiece is made negative.

The wire electrical discharge machining apparatus in accordance withthis invention is a wire electrical discharge machining apparatus formachining a workpiece by supplying discharge energy to a gap between awire electrode and the workpiece by working-electric-power supplyingmeans having a dc power supply and switching means for switching anoutput voltage of the de power supply, and by relatively moving the wireelectrode and the workpiece by positioning means, comprising:controlling means which, during machining in the atmosphere or duringmachining effected while supplying a pressure gas to the gap by gassupplying means, controls the turning on and off of the switching meansso as to apply to the gap a voltage of reverse polarity in which thepolarity of the wire electrode is made positive and the polarity of theworkpiece is made negative.

In addition, the wire electrical discharge machining apparatus inaccordance with this invention is a wire electrical discharge machiningapparatus for machining a workpiece by supplying discharge energy to agap between a wire electrode and the workpiece by working-electric-powersupplying means having a dc power supply and switching means forswitching an output voltage of the dc power supply and capable ofapplying to the gap voltages of both normal and reverse polarities, andby relatively moving the wire electrode and the workpiece by positioningmeans, comprising: mean-voltage detecting means for detecting a meanvoltage in the gap; and controlling means which, during machining in aworking fluid, controls the turning on and off of the switching means soas to apply to the gap the voltage of reverse polarity in which thepolarity of the wire electrode is made positive and the polarity of theworkpiece is made negative and the voltage of normal polarity in whichthe polarity of the wire electrode is made negative and the polarity ofthe workpiece is made positive, and so as to set a value of a meanvoltage in the gap detected by the mean-voltage detecting means tosubstantially 0 V, and which, during machining in the atmosphere orduring machining effected while supplying a pressure gas to the gap bygas supplying means, controls the turning on and off of the switchingmeans so as to apply to the gap a voltage of reverse polarity.

Since the method of and the apparatus for wire electrical dischargemachining are arranged as described above, advantages are offered inthat the surface roughness of the machined surface of the workpiece canbe made smoother, and that it becomes possible to cope with applicationsin which higher accuracy is required and applications in which highquality of the workpiece surface is required.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram illustrating an example of a method ofwire electrical discharge machining in accordance with an embodiment ofthis invention;

FIG. 2 is a block diagram illustrating the configuration of a wireelectrical discharge machining apparatus in accordance with theembodiment of this invention;

FIG. 3 is a diagram illustrating an example of the voltage waveform inthe gap between a wire electrode and a workpiece in the wire electricaldischarge machining apparatus in accordance with the embodiment of thisinvention;

FIG. 4 is a diagram illustrating the results of an experiment which wasconducted to make a comparison between wire electrical dischargemachining in water and wire electrical discharge machining in theatmosphere and a comparison between wire electrical discharge machiningwith normal polarity and wire electrical discharge machining withreverse polarity;

FIG. 5 is an explanatory diagram of a mechanism of electrical dischargemachining;

FIG. 6 is an explanatory diagram illustrating an example of themachining process of wire electrical discharge machining; and

FIG. 7 is a diagram illustrating examples of the voltage and the currentwaveform in the gap between the wire electrode and the workpiece.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is an explanatory diagram illustrating an example of a method ofwire electrical discharge machining in accordance with an embodiment ofthis invention. In the drawing, reference numeral 1 a denotes a wireelectrode; 2, a workpiece; 4 a, water, i.e., a working liquid; 6, aninitial hole; and 7, a gas such as air., oxygen, nitrogen, hydrogen, aninert gas, an insulating gas, or the like. The part (a) of FIG. 1 showsa first cut which is rough machining, and the part (b) of FIG. 1 shows asecond cut which is finish machining after the rough machining. Theterms ‘the first cut and the second cut’ are for the sake ofconvenience, and it does not necessarily follow that wire electricaldischarge machining is completed in two steps of machining. In machiningin which the accuracy required for the workpiece is high, there arecases where machining is effected in as many as seven or eight steps.

Next, a description will be given of an outline of the machining method.The first cut shown in the part (a) of FIG. 1 is machining in which thewire electrode 1 a is passed through the initial hole 6 and the wireelectrical discharge machining 2 is bored. In the first cut, since thesurface roughness and accuracy of the machined surfaces of the workpieceare finished in subsequent machining, strict surface roughness andaccuracy are not required so much, and it is particularly important toincrease the machining speed to improve productivity. In the same way asin FIG. 6 concerning the background art, machining is effected with thewater 4 a, i.e., the working fluid, interposed in the gap between thewire electrode 1 a and the workpiece 2.

In ordinary wire electrical discharge machining, machining proceeds inthe working fluid even after the first cut. However, since there areproblems in such as the vibration of the wire electrode 1 a as shown inthe background art, the ordinary wire electrical discharge machining isnot suitable for high-accuracy machining.

In the second cut shown in the part (b) of FIG. 1, which is finishmachining, machining is effected not in the working fluid 4 a but in thegas 7 to suppress the vibration of the wire electrode 1 a and improvethe machining accuracy. It is possible to suppress the vibration and thelike of the wire electrode 1 a by such aerial wire electrical dischargemachining, as shown below.

Namely, since the electrostatic force acting between the wire electrode1 a and the workpiece 2 when a voltage is applied across the gap isproportional to the dielectric constant of the gap, if a calculation ismade under the assumption that the gap distance is the same, in the casewhere the interposed object in the gap is the gas 7, the electrostaticforce becomes one-tenths (for example, the dielectric constant is thesmallest in a vacuum, and in water it is about 80-fold that in thevacuum) as compared with the case where the interposed object in the gapis the water 4 a. In addition, since the explosive force of vaporizationdue to the electric discharge is generated by the liquid interposed inthe gap, in the case where only the gas 7 is present in the gap, thewire electrode 1 a remains practically unaffected by the explosive forceof vaporization. Accordingly, since the vibration and the like of thewire electrode 1 a can be suppressed, the shape accuracy and surfaceroughness of the machined surfaces of the workpiece improve.

FIG. 2 is an explanatory diagram illustrating the configuration of awire electrical discharge machining apparatus in accordance with theembodiment of this invention, and illustrates an example of theconfiguration which is capable of realizing aerial wire electricaldischarge machining such as the one shown in the part (b) of FIG. 1. InFIG. 2, reference numeral 1 a denotes the wire electrode; 2, theworkpiece; 7, a gas such as air, oxygen, nitrogen, hydrogen, an inertgas, an insulating gas, or the like; and 8, a wire bobbin. Numerals 9 aand 9 b denote gas supplying means for supplying the gas 7 to the gapbetween the wire electrode 1 a and the workpiece 2. Numeral 10 denotes acapstan roller; 11, a pinch roller; 12, an X table for driving theworkpiece 2 in a horizontal direction (in the X direction); 13, a Ytable for driving the workpiece 2 in a horizontal direction (in the Ydirection); 14, an X-axis servo amplifier for controlling anunillustrated drive motor for driving the X table 12; 15, a Y-axis servoamplifier for controlling an unillustrated drive motor for driving the Ytable 13; 16, a working-electric-power supplying means; 17, a dc powersupply for supplying a dc voltage to the gap between the wire electrode1 a and the workpiece 2; and 18, a gap resistor. Numerals 19 a, 19 b, 19c, and 19 d denote switching means such as field effect transistors foreffecting the switching of the dc voltage. Numeral 20 denotes acontrolling means for effect the on-off control and the like of theswitching means 19 a, 19 b, 19 c, and 19 d, and numeral 21 denotes amean-voltage detecting means for detecting the mean voltage in the gap.

Next, a description will be given of the operation. The wire electrode 1a is clamped and pulled by the capstan roller 10 and the pinch roller 11to cause the wire electrode 1 a to travel, and the wire electrode 1 aand the workpiece 2 are caused to oppose each other. While the gas 7 isbeing supplied to the gap between the wire electrode 1 a and theworkpiece 2 by the gas supplying means 9 a and 9 b, working electricpower, i.e., the discharge energy, is supplied to the gap by theworking-electric-power supplying means 16. The wire electrode 1 a andthe workpiece 2 are moved relative to each other by the X table 12, theY table 13, and the like, which are the positioning means, to effectfinish machining. Control of the relative positioning of the wireelectrode 1 a and the workpiece 2 by the positioning means, control ofthe electric machining conditions, and the like are administered by thecontrolling means 20. The gas supplying means 9 a and 9 b can berealized by, for example, such as supplying a pressure gas after formingnozzles around the wire electrode la. By supplying such a pressure gasto the gap, it is possible to prevent the machining debris removed bythe discharge from adhering to the wire electrode and the work piecesurfaces. In addition, aerial wire electrical discharge machining canalso be effected in the atmosphere without using such gas supplyingmeans 9 a and 9 b.

It suffices if in a state in which the water 4 a, i.e., the workingfluid, is stored in an unillustrated working tank used in wireelectrical discharge machining in an ordinary working fluid, and theworkpiece 2 is immersed in it, the first cut, which is rough machining,in the part (a) of FIG. 1 is effected. It then suffices if after thewater 4 a in the working tank is discharged, the second cut, which isfinish machining, in the part (b) of FIG. 1 is effected in a gas byusing the configuration of the wire electrical discharge machiningapparatus such as the one shown in FIG. 2.

In addition, the wire electrical discharge machining apparatus inaccordance with the embodiment of this invention adopts a switchingcircuit of a bridge formation which is capable of applying voltages ofnormal and reverse polarities to the gap between the wire electrode 1 aand the workpiece 2, as shown in FIG. 2. Accordingly, if the switchingmeans 19 a and 19 b are turned on and the switching means 19 c and 19 dare turned off by the controlling means 20, it is possible to apply tothe gap a voltage of normal polarity in which the polarity of the wireelectrode 1 a is negative and the polarity of the workpiece 2 ispositive. On the other hand, if the switching means 19 c and 19 d areturned on and the switching means 19 a and 19 b are turned off by thecontrolling means 20, it is possible to apply to the gap a voltage ofreverse polarity in which the polarity of the wire electrode 1 a ispositive and the polarity of the workpiece 2 is negative.

FIG. 3 shows an example of the voltage waveform in the gap between thewire electrode 1 a and the workpiece 2 in the wire electrical dischargemachining apparatus in accordance with the embodiment of this invention.In the drawing, V denotes a gap voltage, and t denotes time.

The part (a) of FIG. 3 shows an example of the gap voltage waveform in acase where wire electrical discharge machining is effected with thewater 4 a, i.e., the working fluid, interposed in the gap. On-offcontrol of the switching means 19 a, 19 b, 19 c, and 19 d is provided bythe controlling means 20 shown in FIG. 2, thereby applying to the gapthe voltage of reverse polarity in which the polarity of the wireelectrode la is positive and the polarity of the workpiece 2 is negativeand the voltage of normal polarity in which the polarity of the wireelectrode 1 a is negative and the polarity of the workpiece 2 ispositive. In addition, the pulse widths of gap voltage pulses of bothnormal and reverse polarities are adjusted so that the value of the meanvoltage in the gap detected by the mean-voltage detecting means 21 shownin FIG. 2 becomes substantially 0 V. Thus it is possible to suppresselectrolytic corrosion in which ions of the electrode on the plus sideare eluted into the fluid and corrosion progresses.

The part (b) of FIG. 3 shows an example of the gap voltage waveform in acase where wire electrical discharge machining is effected in the gas 7.On-off control of the switching means 19 a, 19 b, 19 c, and 19 d isprovided by the controlling means 20 shown in FIG. 2, thereby effectingmachining by the discharge with the reverse polarity. In the part (b) ofFIG. 3, the mean voltage in the gap cannot be set substantially to 0 Vas in the case of the part (a) of FIG. 3, but since the fluid is notpresent in the gap in the part (b) of FIG. 3, electrolytic corrosiondoes not occur.

In addition, the part (b) of FIG. 3 shows an example of a case in whichlarge quiescent time is provided between groups of voltage pulses in thegap. However, this invention is not limited to such a case, and sincethe electrostatic force itself is small in aerial electrical dischargemachining, it is also possible to set the quiescent time to be shorter.

FIG. 4 is a diagram illustrating the results of an experiment which wasconducted to make a comparison between wire electrical dischargemachining in water and wire electrical discharge machining in a gas anda comparison between wire electrical discharge machining with normalpolarity and wire electrical discharge machining with reverse polarity.Q (μC) denotes the amount of charge transferred in a single discharge,and D (μm) denotes the depth of a discharge crater in the machinedsurface of the workpiece in the single discharge. The surface roughnessof the machined surface of the workpiece is defined by the depth of thedischarge crater.

As for the symbols in the drawing, ▪ shows a case in which wireelectrical discharge machining was performed with normal polarity inwater; □, with reverse polarity in water; ●, with normal polarity in agas; ◯, with reverse polarity in a gas. These symbols show the resultsof measurement concerning a discharge crater formed in a singledischarge. From FIG. 4, it can be appreciated that the amount of chargetransferred and the depth of the discharge crater (i.e., the surfaceroughness of the machined surface of the workpiece) are stronglycorrelated to each other. From the observation of the craters of thesingle discharge, it can be understood that the discharge crater issmaller for □ than for ▪, and that the discharge crater is smaller for ◯than for ●. Namely, it can be appreciated that, in the discharge of thesame amount of charge, the discharge crater is smaller in the case ofthe reverse polarity than in the case of the normal polarity. Inaddition, it can also be understood that the discharge crater is thesmallest in the case of ◯ (reverse polarity; in a gas).

Accordingly, if aerial wire electrical discharge machining is effectedby applying to the gap the voltage of reverse polarity in which thepolarity of the wire electrode 1 a is positive and the polarity of theworkpiece 2 is negative, the surface roughness of the machined surfaceof the workpiece can be made smoothest.

Ordinary wire electrical discharge machining in water (corresponding tothe part (a) in FIG. 1) is effected by the working-electric-powersupplying means 16 capable of applying voltages of both normal andreverse polarities to the gap between the wire electrode 1 a and theworkpiece 2. In this case, the roughest discharge crater occurs in thecase of the normal polarity in which the polarity of the wire electrode1 a is negative and the polarity of the workpiece 2 is positive. Thisirregularity becomes dominant with respect to the surface roughness ofthe machined surface of the workpiece. Then, if aerial wire electricaldischarge machining (corresponding to the part (b) of FIG. 1) iseffected by supplying to the gap the voltage of reverse polarity inwhich the polarity of the wire electrode 1 a is positive and thepolarity of the workpiece 2 is negative through on-off control of theswitching means 19 a, 19 b, 19 c, and 19 d by the controlling means 20,it becomes possible to cope with such as applications in which higheraccuracy is required or applications in which high quality of theworkpiece surface is required.

Industrial Applicability

As described above, the method of and the apparatus for wire electricaldischarge machining in accordance with this invention are particularlysuitable for high-accuracy electrical discharge machining operation andhigh-quality electrical discharge machining operation.

1. A method of wire electrical discharge machining for machining aworkpiece by generating electric discharge in a gap between a wireelectrode and the workpiece, characterized in that machining is effectedby supplying to the gap a voltage of reverse polarity in which thepolarity of the wire electrode is made positive and the polarity of theworkpiece is made negative, in the atmosphere or while supplying apressure gas to the gap.
 2. A method of wire electrical dischargemachining for machining a workpiece by generating electric discharge ina gap between a wire electrode and the workpiece, characterized bycomprising: a first step of effecting rough machining in a workingfluid; and a second step of effecting finish machining in the atmosphereor while supplying a pressure gas to the gap, wherein, in the secondstep, machining is effected by supplying to the gap a voltage of reversepolarity in which the polarity of the wire electrode is made positiveand the polarity of the workpiece is made negative.
 3. A wire electricaldischarge machining apparatus for machining a workpiece by supplyingdischarge energy to a gap between a wire electrode and the workpiece byworking-electric-power supplying means having a dc power supply andswitching means for switching an output voltage of the dc power supply,and by relatively moving the wire electrode and the workpiece bypositioning means, characterized by comprising: controlling means which,during machining in the atmosphere or during machining effected whilesupplying a pressure gas to the gap by gas supplying means, controls theturning on and off of the switching means so as to apply to the gap avoltage of reverse polarity in which the polarity of the wire electrodeis made positive and the polarity of the workpiece is made negative. 4.A wire electrical discharge machining apparatus for machining aworkpiece by supplying discharge energy to a gap between a wireelectrode and the workpiece by working-electric-power supplying meanshaving a dc power supply and switching means for switching an outputvoltage of the dc power supply and capable of applying to the gapvoltages of both normal and reverse polarities, and by relatively movingthe wire electrode and the workpiece by positioning means, characterizedby comprising: mean-voltage detecting means for detecting a mean voltagein the gap; and controlling means which, during machining in a workingfluid, controls the turning on and off of the switching means so as toapply to the gap the voltage of reverse polarity in which the polarityof the wire electrode is made positive and the polarity of the workpieceis made negative and the voltage of normal polarity in which thepolarity of the wire electrode is made negative and the polarity of theworkpiece is made positive, and so as to set a value of a mean voltagein the gap detected by the mean-voltage detecting means to substantially0 V, and which, during machining in the atmosphere or during machiningeffected while supplying a pressure gas to the gap by gas supplyingmeans, controls the turning on and off of the switching means so as toapply to the gap a voltage of reverse polarity.